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From: "i at maskray dot me" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug dynamic-link/27924] New: ld.so: Support DT_RELR relative
 relocation format
Date: Thu, 27 May 2021 18:14:01 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D27924

            Bug ID: 27924
           Summary: ld.so: Support DT_RELR relative relocation format
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
  Target Milestone: ---

Position independent executables and shared objects built with
-Bsymbolic/-Bsymbolic-functions may have many relative relocations (instead=
 of
absolute relocations (e.g. R_X86_64_64)).

(The number of R_*_RELATIVE is the dominating factor between a PDE and a PI=
E in
terms of their size differences.)

Relative relocations can be packed in a compact format to save a lot of spa=
ce.
See the last few messages on
https://groups.google.com/g/generic-abi/c/bX460iggiKg

An Elf64_Rela takes 24 bytes. 64 consecutive R_*_RELATIVE take 24*64 bytes.=
 In
RELR, this just needs 8*2 bytes.

DT_RELR has support in the Linux kernel (arch/arm64/Kconfig ARCH_HAS_RELR)
since 2019
https://git.kernel.org/linus/5cf896fb6be3effd9aea455b22213e27be8bdb1d

SHT_RELR/DT_RELR has dumper support in llvm-readobj/llvm-readelf.=20
ld.lld --pack-dyn-relocs=3Drelr can encode R_*_RELATIVE in RELR.

GNU ld feature request: https://sourceware.org/bugzilla/show_bug.cgi?id=3D2=
7923

--=20
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